Log Message:
Update to 0.15.0

* Servers can be assigned different weights to account for
  differing capacity. New -W command-line option. New penctl
  commands:
  server S weight W       (assign weight to server)
  weight                  (use weight for server selection)
  no weight               (do not use weight for server selection)

  Cleaned up the logic in add_client so the weighted server
  selection can be used without client tracking.

* Some performance enhancing changes:

        New variable connections_used remembers the number of used
        slots in conns[]. It is incremented by store_conn and
        decremented by close_conn. This allows the main loop to
        only accept new connections if there are empty slots in
        conns[], which is much better than accepting the connection
        only to immediately close it because we can't handle it.

        New variable connections_last remembers the last used slot
        in conns[]. This allows us to scan for empty slots much faster
        in store_conn when there are many simultaneous connections.

* Documented the procedure to change FD_SETSIZE on Linux
 in INSTALL.

* Documented the include command in the penctl manpage.

* Fixed SSL so it works in nonblocking mode, except that
  it doesn't work anyway.
  Moved listenfd and ctrlfd out of main.

* Highly experimental SSL code in pen.c. Updated manpage
  with the new options. Added https example to HOWTO.